[canvaskit] Speculative fix for flaky dommatrix test

There's some flakes on the _dommatrix test. This adds an extra assertion
to maybe shed some light on that flake.

Change-Id: I548d3294c8f3c620a9702e4d4889da745a2d91fe
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/293764
Reviewed-by: Kevin Lubick <kjlubick@google.com>
3 files changed